asteroid

This representation of the physical properties of
interesting asteroids includes most of the asteroids larger than about
200 km in diameter. They are shown in their correct relative sizes
and shapes (the limb of Mars is shown for comparison). The bodies
are positioned at their correct relative distances from the Sun. Asteroids
located near the top or bottom of the diagram occupy relatively eccentric
or inclined orbits (or both), while those shown near the ecliptic
plane move in relatively circular, noninclined orbits.

Asteroid Gaspra taken by NEAR-Shoemaker
as it flew by.

Artwork of a collision between asteroids.

An asteroid is a rocky or metallic object, generally smaller than a planet but bigger than a meteoroid, that orbits
the Sun or another star. Asteroids are also called minor planets. What had been the largest
asteroid, Ceres, has been classified
as a dwarf planet.

More than 20,000 asteroids have been given official designations, most of
them in the asteroid belt between the orbits
of Mars and Jupiter.
Asteroids outside this belt include the Trojans,
which share Jupiter's orbit, and the near-Earth
asteroids. A number of different asteroid groups have been distinguished
on the basis of their similar orbital characteristics. Other sub-planetary
objects, considered distinct from asteroids, are Centaurs, Kuiper Belt objects, and comets,
though classification is ambiguous in some cases. Some of the smaller moons
in the solar system appear to be captured asteroids, including the two moons
of Mars, and a number of the outer moons of the four gas giants.

Size of asteroids

Asteroids range in size from a few meters to over 900 kilometers across, and vary
greatly in composition. Although none is visible to the naked eye, many
can be seen at times with binoculars or small telescopes, including the
four largest: 1 Ceres, 2 Pallas,
4 Vesta, and 10 Hygiea.
Thirty known asteroids exceed 200 kilometers in diameter and the census of asteroids
larger than 100 kilometers in diameter is believed to be virtually complete. In
the 10 to 100 kilometer range, probably about half await discovery. However, of
the estimated one million asteroids bigger than 1 kilometer across, only a tiny
percentage is known. The total mass of all the asteroids, most of which
is concentrated in the main belt, is about one-twentieth that of the Moon
and about three times that of Ceres. Some asteroids, such as Ceres, Pallas,
and Vesta, are nearly spherical; others, like 15 Eunomia (see Eunomia
family), 107 Camilla, and 511 Davida, are quite elongated; still
others, such as 4769 Castalia, 216 Kleopatra, and 4179 Toutatis,
have bizarre shapes.

Companions
of asteroids

Several asteroids, including 243 Ida, 45 Eugenia, and 762 Pulcova, are known to
have small moons of their own. The discovery of these moons is important
because it enables an accurate determination of the parent asteroid's mass
and average density. The density then gives a clue to the asteroid's makeup
– either in terms of composition or of structure. There are also binary
asteroids, such as 90 Antiope and, possibly,
1620 Geographos, in which two components
of roughly equal size orbit each other at very close range. Several asteroids
have been studied by passing space probes, including Ida and 951 Gaspra (by Galileo), and 253 Mathilde and 433 Eros (by NEAR-Shoemaker).

Orbits and rotation of asteroids

Most asteroids move in orbits that are somewhat more inclined and eccentric
than those of the major planets (with the exception of Pluto)
– the orbit of an average main-belt asteroid being inclined at about
10° to the plane of the ecliptic with an eccentricity of about 0.15.
But some asteroids, such 3200 Phaethon and 944 Hidalgo, have highly inclined and/or elliptical paths, suggesting
they may be defunct cometary nuclei. Rotational periods of asteroids range
from 2.3 hours to 48 days, but in more than 80% of cases are 4 to 20 hours.

Albedo

Albedos vary from just under 0.02 to over 0.5,
with the majority of asteroids tending toward the lower (dark) end of this
range. Low-albedo asteroids are generally found in the outer half of the
asteroid belt, while higher-albedo objects tend to occupy the inner half.
This fact stems from compositional differences, which in turn are related
to how far from the Sun asteroids of different types formed.

Origin of asteroids

Asteroids are thought to be the remnants of a stillborn planet. According
to this idea, the newborn Jupiter gravitationally scattered nearby large
planetesimals – accreting lumps of matter in the embryonic stage of
planet-formation – some of which may have been as massive as Earth
is today. Some of these big planetesimals strongly perturbed the orbits
of the planetesimals in the region of the asteroid belt, raising their mutual
velocities to the average 5 km/s seen today. As a result, what had been
mild accretionary collisions in the future belt region became catastrophic
disruptions. Only objects larger than about 500 km in diameter could have
survived 5 km/s collisions with objects of comparable size. Ever since,
the asteroids have been collisionally evolving so that, with the exception
of the largest, most present-day asteroids are either remnants or fragments
of past impacts.

While breaking down larger asteroids into smaller ones, collisions expose
deeper layers of asteroidal material. If asteroids were compositionally
homogeneous, this would have no noticeable result. Some of them, however,
became differentiated; in other words, after they formed from primitive
material in the solar nebula, they were heated (by radioactive decay or
other means) to the point where their interiors melted and geochemical processes
occurred. In some cases, temperatures became high enough for iron to form.
Being denser than other materials, the iron sank to the center, forming
an iron core and forcing basaltic lavas to the surface. At least one asteroid
with a basaltic surface, Vesta, survives
to this day. Other differentiated asteroids were disrupted by collisions
that stripped away their crusts and mantles and exposed their iron cores.
Still others may have had only their crusts partially stripped away, which
exposed surfaces such as those visible today on the A-, E-, and R-class
asteroids.

Asteroid impacts

Collisions gave rise to the Hirayama
families and at least some of the planet-crossing asteroids. Tiny fragments
from the latter enter Earth's atmosphere to become sporadic meteors,
while larger pieces survive passage through the atmosphere to end up as meteorites. The very largest produce craters
such as the Barringer Crater, and
one may have been responsible for the extinction of the dinosaurs. Luckily,
collisions of this sort are rare. According to current estimates, a few
asteroids of 1-km diameter collide with Earth every 1 million years. Past
collisions between asteroids and the Earth appear to have played a crucial
role in the evolution of life on this planet (see cosmic
collisions, biological effects). In particular, the impact of an asteroid
about 65 million years ago caused a mass
extinction in which the last of the dinosaurs were wiped out (see Cretaceous-Tertiary
Boundary).

The detection and tracking of near-Earth objects that might collide with
the Earth in the future is receiving increasing attention. In addition,
a number of space missions to investigate asteroids more closely, including
the return of samples, have taken place or are underway. These include NEAR-Shoemaker, MUSES-C, and Deep
Space 1.

Asteroid names

Following its discovery, an asteroid is given a preliminary designation
that consists of the year of discovery, an upper case letter to indicate
the half-month in that year (A=Jan 1-15, B=Jan 16-31, ..., Y=Dec 16-31,
the letter "I" being omitted), and a second upper case letter in sequence.
When this sequence of 25 letters has been completed, it is repeated and
followed by a sequential number. A permanent designation, consisting of
a number and a name, is given to asteroids whose orbits have been accurately
determined. The number represents the order of discovery, starting from
1 Ceres. The name is generally proposed by the discoverer and submitted
to the International Astronomical Union for
approval.

Asteroid types

An asteroid type is any of a number of categories that an asteroid can
be placed in based on its reflectance spectrum and albedo, which are indicators
of surface composition. The distribution of the various typees throughout
the asteroid belt is highly structured, suggesting that many asteroids formed
at or near their present distances from the Sun and are representative of
the composition of the solar nebula (not including hydrogen and helium)
at these locations. Class S (silicaceous) asteroids are more prevalent in
the inner part of the main asteroid belt, giving way to Class C (carbonaceous)
in the middle and outer parts of the belt. Together, these two types account
for about 90% of the asteroid population. Class M (metal) objects are especially
concentrated in the middle of the belt, while dark reddish asteroids of
classes P and D become commoner at the outer edge of the belt and beyond.

The most widely used taxonomic scheme, proposed by American astronomer David
Tholen and Italian astronomer Maria Barucci in 1989, identifies 14 different
types – A, B, C, D, E, F, G, M, P, Q, R, S, T, and V. However, there is
disagreement, on the one hand, about whether all of the Tholen-Barucci classes
are distinct and, on the other, about whether additional categories are
needed. To add to the confusion, because most meteorites are known to be
fragments of asteroids, there is an on-going effort to match meteorite classification
with asteroid classification. This is difficult because whereas meteorites
can be analyzed in the lab to determine their exact chemical composition
and petrologic type, most of the information about asteroids comes from
remote spectroscopy.

Asteroid groups

An asteroid group is a collection of asteroids that move in generally similar
orbits. Within a group there may be one or more families of asteroids whose
orbital characteristics are so much alike that they almost certainly came
from the breakup of a single parent body. Most groups and families (generally
named after the first-discovered member) are found in or near the main asteroid
belt. They include the Hungaria group, Flora family, Nysa-Polana
family, Phocaea group, Koronis
family, Eos family, Themis
family, Cybele group, Alinda
group, and Hilda group. The three
largest families (Eos, Koronis, and Themis) have been determined to be compositionally
homogeneous. If the asteroids belonging to them are considered to be fragments
of a single parent body, then these parent bodies probably had diameters
of 100 to 300 km. The smaller families have not been as well studied because
they have fewer and smaller members; however, it is known that some of the
smaller families are compositionally inhomogeneous and that, at least in
some cases, what are observed are pieces of a geochemically differentiated
parent body. Beyond the main belt are the Jupiter Trojans,
while inside the belt are the Mars-crossers and Mars Trojans. Even closer to the
Sun are three groups of near-Earth
asteroids: the Amor group, Aten
group, and the Apollo group.